Immune deficiency causes significant morbidity and mortality in hematopoietic cell transplant (HCT) recipients. The clinical significance is especially evident when utilizing high doses of cytotoxic conditioning, donor HSC sources that are HLA-mismatched or contain low numbers of mature T lymphocytes, as would be the case with umbilical cord blood (UCB) transplants. We have observed a high incidence of severe or fatal intracellular and DNA viral (CMV; EBV; HHV6; adenovirus) infections in UCB transplant recipients. A major cause is loss of thymopoietic capacity, and impaired T cell recovery as a result of age, chemoradiotherapy or graft-versus-host-disease. Thymopoiesis depends on the interaction of the thymic stroma-derived receptors and ligands. Damage to thymic epithelial cells (TECs) by pre-transplant conditioning impairs the generation of mature T cells following transplant and predisposes the recipient to infections. Our central hypothesis is that thymic microenvironmental injury is the major limiting factor for slow T cell reconstitution and function in UCB transplant recipients, indicated by the predisposition for late infections. TEC differentiation, proliferation, and survival are controlled by both cell intrinsic and extrinsic factors. Thymocyte precursors and TECs engage in cross-talk such that bidirectional signaling mechanisms provided by and to both thymocytes and TECs are essential for their mutual proliferation and survival. Our preliminary data in mice indicate that there is a direct correlation between the number of mature thymocytes and TECs, especially those located in the thymic medulla, which serves as the primary site of negative selection and thymic egress into the periphery. With the operational hypothesis that the rapidity of recovery of effective thymopoiesis is limited both by ineffective :TEC cross-talk (aim 1) and the relative paucity of endogenous TECs that escape conditioning regimens (aims 1, 2). We will pursue two approaches (specific aims) to overcome the quantitative and qualitative defect in TEC support of thymopoiesis. Aim 1. To determine whether TEC regeneration and thymopoietic recovery is limited by inadequate cross-talk between thymocyte precursors and TECs post-HSCT. Aim 2. To devise and test novel TEC replacement strategies based upon TEC developmental cues and using inducible pluripotent stem cell technology.